Ink jet printing mechanisms use pens that shoot droplets of colorant onto a printable surface to generate an image. Such mechanisms may be used in a wide variety of applications, including computer printers, plotters, copiers, and facsimile machines. For convenience, the concepts of the invention are discussed in the context of a printer. An ink jet printer typically includes a print head having a multitude of independently addressable firing units located on a silicon die, along with connecting circuitry. Each firing unit includes an ink chamber connected to a common ink source, and to an ink outlet nozzle. A transducer within the chamber provides the impetus for expelling ink droplets through the nozzles. In thermal ink jet printers, the transducers are thin film firing resistors that generate sufficient heat during application of a brief voltage pulse to vaporize a quantity of ink sufficient to expel a liquid droplet.
Its it important to maintain a controlled temperature of the die in thermal ink jet printers. Below a normal operating temperature, resistor firing characteristics are affected, and ink viscosity impairs normal fluid flow. Consequently, overall printing performance and uniformity are impaired, and thermal control is required. Thermal control is also required to detect excessive pen temperatures, such as may occur in cases in which extremely demanding continued printing occurs at high speed, or when a depletion of the ink supply goes undetected. Such excessive temperatures may cause a catastrophic pen failure due to thermal runaway, requiring costly component replacement or service.
Existing ink jet printers monitor die temperature by use of a thin film sensor resistor on the die. The printer is connected to the sensor resistor via a line on the interconnect set used also to provide power and printing data to the die. The printer circuitry includes what is essentially a digital ohmmeter that reads the resistance of the sensor resistor, and infers the resistor temperature based upon the principle that resistance is proportional to temperature. This system has limited accuracy because the sensor resistor provides only a weak analog signal voltage that changes only slightly in response to temperature, with a voltage change of 5 mV/.degree. C. being typical. This a particular concern because the numerous other lines of the interconnect and flex circuit connecting the printer to the die are very electrically noisy, with currents of up to 8 A undergoing high speed hard switching during normal printer operations. Thus, the relatively faint voltage indicating temperature may be distorted or lost in the EFI noise generated during printing. In addition, the printer operations to measure the die temperature may require additional computing overhead, which may slow or divert controller resources from the printing operation.
The present invention overcomes the limitations of the prior art by providing a thermal ink jet print head with numerous firing elements on a die, and a temperature sensor on the die with a sensor voltage output proportional to a sensed temperature. A digital to analog converter has a digital input and an output voltage proportional to the value of a digital word received by the digital input, and a comparator has a first input connected to the sensor voltage output and a second input connected to the converter voltage output. The comparator generates an equivalency signal when the converter output voltage exceeds the sensor output voltage. The print head may have a temperature controller that compares the digital word to a preselected temperature threshold value to determine if the temperature is within a selected range, and which changes the temperature of the die in response to a determination that the temperature is outside of the selected range.